Generally, the operating characteristics of a conventional engine are influenced by the operating characteristics of the cams which actuate the valves of the engine. For example, if the shape of the cams is selected so as to produce a high torque when the engine including the cams rotates at a low speed, the engine cannot produce a sufficiently high torque at a high rotating speed. On the other hand, if the shape of the cams is selected so that an engine having the cams can produce a high torque at a high rotating speed, the output of the engine is decreased when the engine speed is low. As a result, a conventional engine cannot always produce a high torque for various rotating speeds of the engine.
To increase the torque characteristics of an engine for various rotating speeds of the engine, U.S. Pat. No. 3,878,822, issued to Beal, discloses a variable valve event engine comprising: a rocker shaft disposed along the engine; a rocker arm for actuating a valve of the engine, which arm is pivoted swingably around and slidably along the rocker shaft; a camshaft arranged parallel to the rocker shaft and synchronized with a crankshaft of the engine; and a plurality of adjacent cams fixed in such an arrangement on the camshaft as to have profiles different from each other, wherein by sliding the rocker arm along the rocker shaft, the valve is selectively actuated by means of one of the cams.
However, the engine disclosed in U.S. Pat. No. 3,878,822 has several disadvantages in actual operation. For example, since the rocker arm is moved by means of an electromagnetic solenoid, the force for moving the rocker arm is small. In addition, since the rocker shaft itself is moved by the electromagnetic solenoid, the moving speed of the rocker arm is very slow.
To overcome such disadvantages, in U.S. Pat. No. 4,253,434 which is assigned to the same assignee as the present application, a variable valve event engine is disclosed. The engine comprises: a rocker shaft disposed along the engine; a rocker arm for actuating a valve of the engine, the rocker arm being pivotably and slidably disposed on the rocker shaft; a hydraulic member for moving the rocker arm along the rocker shaft; a camshaft disposed parallel to the rocke shaft and synchronized with a crank shaft of the engine; a plurality of cams having cam profiles different from each other fixed on the cam shaft so as to be adjacent to each other; a rocker arm slide shaft disposed parallel and longitudinally movable with respect to the rocker shaft, and; a spring connecting the rocker arm slide shaft to the rocker arm for transmitting movement of the rocker arm slide shaft to the rocker arm; wherein by sliding the rocker arm along the rocker shaft, the valve is selectively actuated by means of one of the cams.
In such a variable valve event engine as described in the above-mentioned United States Patents, the rocker arm must be rapidly moved while the rocker arm is positioned at base circular portions which are common to the adjacent two cams having different cam profiles, so that abrasion and damage of the rocker arm and cams is prevented and so that the smooth switching of the rocker arm can be effected. For this purpose, the above-mentioned U.S. Pat. No. 4,253,434 discloses; a spring urging member for retaining a predetermined amount of energy which is utilized to move the rocker arm rapidly, and; a stopper guide positioned between two adjacent cams which prevents the rocker arm from moving at a position different from the common base circular portion.
The inventors of the present invention confirmed that in many cases the above-mentioned spring urging member and the stopper guide are very effective, but that under some driving conditions they observed an unfavorable phenomenon occuring in that the movement of the rocker arm cannot be completed during the common base circular portions of the cams. Through careful investigation, the inventors found that such unfavorable phenomenon occurs just after the engine is started and when the viscosity of the lubricant within the lubricating system in the engine is high since the temperature of the lubricant is low. More specifically, the phenomenon is caused by the fact that the moving speed of the rocker arm is low since the viscosity of the lubricant is high and, accordingly, the rocker arm cannot move rapidly between the cams.